\documentclass{standalone}
\usepackage{tikz,ctex}
\usetikzlibrary{patterns}
\begin{document}
\footnotesize
  \begin{tikzpicture}[>=stealth,scale=0.8]
    \draw[thick](-1.5,-0.75)rectangle(1.5,0.75);
    \draw[thick](-0.25,0.25)arc(180:0:0.25)--(0.25,-0.25)arc(360:180:0.25)--cycle;
    \draw[line width=1.5pt](-2,0.75)arc(150:210:1.5);
    \draw[thin,<-](-2.201,0)--++(-0.2,0);
    \draw[thin,<-](-2,0)--++(0.2,0)node[midway,above]{$f$};
    \draw[thin](-2.201,0)--(-2,0);
    \draw[thin](1.5,0.8)--++(0,0.3)(-1.5,0.8)--++(0,0.3);
    \draw[thin,<->](-1.5,1.0)--(1.5,1.0)node[midway,sloped,above]{$L$};
    \draw[thin](1.55,0.75)--++(0.5,0)(1.55,-0.75)--++(0.5,0);
    \draw[thin,<->](2.0,-0.75)--(2.0,0.75)node[midway,sloped,above]{$B$};
  \end{tikzpicture}
\end{document}